The present invention relates to battery charging circuits and, in particular, to circuits for charging low-voltage rechargeable batteries, such as lead-acid batteries, from an AC supply voltage. The invention has particular application to the recharging of batteries of the type used in small flashlights and other portable devices.
The present invention is an improvement of the battery charging circuit disclosed in copending application Ser. No. 407,719, filed Aug. 13, 1982, entitled "BATTERY CHARGING FLASHLIGHT CIRCUIT", now U.S. Pat. No. 4,460,863, issued July 17, 1984. That charging circuit is a two-step constant current charging circuit which operates to provide to the battery two separate charging currents, viz., a high current at a relatively low voltage which is terminated when the battery has been charged to a predetermined charge level, and a limited current at a relatively high voltage which is continually supplied to the battery. This high voltage charging current operates to facilitate charging of batteries which have been deeply discharged with resulting sulfate buildup on the electrodes. This initial high-voltage charging current helps to breakdown the sulfation and facilitate the acceptance of the full charging current by the battery. After the battery has been substantially fully charged and the low voltage charging current has been switched off, the high voltage current continues to afford a "trickle" charge to maintain the desired charge level without excessively overcharging the battery.
That prior battery charging circuit utilizes a transformer with a tapped secondary to provide the two different charging voltages. The transformer is a relatively expensive and heavy element, which significantly increases the cost and weight of the battery charging circuit. Furthermore, the transformer generates significant amounts of heat inside the product. Also, the transformer supply presents a substantially resistive impedance to the AC line, resulting in an unfavorable power factor.